The present invention relates to a piezoelectric transducer, as well as to a pressure sensor or microphone using such a transducer and which more particularly makes it possible to measure pressure or compressive force values, pressure or compressive force gradient values, as well as bends or curves.
Numerous materials have piezoelectric properties or can have such effects following an appropriate treatment. Piezoelectric materials also have pyroelectric properties. Reference can e.g. be made to polarized ferroelectric ceramics or monocrystalline quartz. More recently, piezoelectric polymers have appeared. The latter are used in the form of flat film metallized on both faces and which are mechanically or non-mechanically oriented and are in all cases polarized by applying an intense electric field (approximately 1 MV/cm) at a temperature equal to or above ambient temperature. As a result of this treatment, the polymer material films acquire pyroelectric and piezoelectric properties making it possible to use them as transducers. Applications of such flat or planar films are as pressure or displacement sensors (microphones, hydrophones, strain gauges, etc) and temperature sensors (pyroelectric sensor for the detection of intruders and for infrared imaging).
Polymers usable for this purpose are e.g. polyvinylidene fluoride (PVF.sub.2), polyvinylchloride (PVC), polyvinylfluoride (PVF), as well as copolymers such as e.g. polytetrafluoroethylene-polyvinylidene fluoride (PTFE-PVF.sub.2) and polyvinylidene fluoride-polytrifluoroethylene (PVF.sub.2 -PTrFE). Flat films of these polymers are conventionally obtained by extrusion blowing, calendering, pressing or solvent evaporation.
The hydrostatic piezoelectric coefficient d.sub.3h of a material can be broken down into three components. For example, for a piezoelectric material sheet like that shown in FIG. 1, the hydrostatic piezoelectric coefficient d.sub.3h of the material is broken down into the sum of three uniaxial piezoelectric coefficients: EQU d.sub.3h =d.sub.31 +d.sub.32 +d.sub.33
In numerous materials, particularly in polyvinylidenefluoride PVF.sub.2 and its copolymers, the signs of the coefficients d.sub.31 and d.sub.32 are opposte to that of d.sub.33. This leads to a low value of three d.sub.3h. In order to befenit from a high sensitivity to hydrostatic stresses, it is e.g. necessary to enclose the active material in a tight steel enclosure and which has a piston applying the pressure uniaxially to the active material in the manner shown in FIG. 2.
A piezoelectric film 10 is placed in a tight enclosure 30 so as to be flat on the bottom thereof. A piston 31 slides in said enclosure 30 and makes it possible to apply a compressive force F to be measured on the piezoelectric film 10. The mechanisms for applying a force F to piston 31 are not shown, but a force F to be applied to the piezoelectric film will necessarily be applied in the displacement direction of piston 31. In this type of arrangement, we have: EQU d.sub.3h =d.sub.33
However, this solution suffers from the disadvantage of requiring precise arrangements, due to the system having to be tight, as well as the loss-free transmission of the forces to be measured and is not suitable for the production of large hydrostatic sensors. Therefore, such a solution is costly.
Another solution, shown in FIG. 3, comprises adhering the piezoelectric film 10 to a rigid support 32 having a very high Young's modulus. Thus, any force which can exist in accordance with the plane of film 1 is opposed by the strength of support 32.
If .nu. is Poisson's ratio of the piezoelectric material, we obtain: ##EQU1##
As .nu.=0.3 for a piezoelectric material, we obtain: EQU d.sub.3h .apprxeq.0.5(d.sub.31 +d.sub.32)+d.sub.33,
which is of interest because d.sub.31 and d.sub.32 have opposite signs to d.sub.33.
However, the latter method is not suitable for the production of large transducers, and this amounts to the same thing, for the production of small transducers by cutting up a large transducer.
Therefore, the present invention proposes a piezoelectric transducer produced in the form of a film and having the same characteristics over its complete area.